Not all of the category 5's sleep 4. Some do not have the pulldown bed over the sofa, so that may be why the website shows them as available, but the reservations agents do not.
Anyway, the rooms are essentially the same, with the obvious difference that the category 8's do not have a verandah. The interiors are the same size. The layout might be a bit different, with the bed near the window and the sofa closer to the door, but I think the amount of living space is the same. I would think that the views would be very similar, except for the breeze that you'd get from the verandah. As for the motion of the ship, I personally do not feel it more from fore to aft, but rather it is more noticable to me the higher I am on the ship (but only slightly so at that). I don't think you'll notice a lot of difference.
If you're dead set on that specific date, my suggestion would be to book the category 8 and waitlist yourself for a category 5 or 6. You could then call back once a week or so to see if anything has opened up. I find the call-back method works better than just waiting for the waitlist to work.
As to whether you'll be happy with the category 8, it's hard to say. I would suggest considering how much of what you liked about the category 5 room had to do with the verandah. I'm of the opinion that being on the ship is preferable to being off of it, so if the category 8 is all you can get, I'd do it. Especially if you have to sail on that date. If the verandah is a must for you, then you might want to consider another sailing, where you could find a cabin with a verandah. Again, there's always the waitlist, which gives you a fair chance of finding an opening and switching from category 8 to 5 (or 6).
Good luck!